WO2001000896A1 - Fe-cr-al alloy - Google Patents

Abstract

The invention relates to a ferritic, oxidation-resistant iron-chrome-aluminium-yttrium-hafnium alloy for thin films and wires. Said alloy has an improved life-span and a reduced oxidation speed at high temperatures, a high hot-drawing resistance and a high specific electric resistance. The alloy has the following composition (in percent by mass): Cr: 16-22, A1: 6-10, Si: 0.02-1.0, Mn: max. 0.5, Hf: 0.02-0.1, Y: 0.02-0.1, Mg: max. 0.1, Ti: max. 0.02, Zr: max. 0.09, SE: max. 0.02, Sr: max. 0.1, Ca: max: 0.1, Cu: max. 0.5 V: max. 0.1 Ta: max. 0.1, Nb: max. 0.1,C: max. 0.03, N: max. 0.01, B: max. 0.01, The residue consists of iron and impurities caused by the melting process.

Description

 FE-CR-AL ALLOY

The invention relates to a ferritic iron-chromium-aluminum-yttrium-hafnium alloy with a long service life and high electrical resistance at high temperatures.

Metallic materials, which are used, for example, as a carrier foil in metallic exhaust gas catalysts or as electrical heating conductors, must have excellent oxidation resistance at high temperatures up to 1200 ° C and must be able to be processed into thin foils or wires. Metal foils with a thickness of 50 μm made of iron-chromium-aluminum alloys with approximately 20 mass% chromium and 5 mass% aluminum are currently used as carrier foils for automotive exhaust gas catalysts, as described by the material number in accordance with DIN 1 .4767 , These materials are protected against destruction by oxidation through the formation of dense, protective aluminum oxide layers.

However, during the cooling from high temperature, the protective oxide layers can flake off due to thermal stresses.

DE-C 370641 5 describes an alloy with increased oxidation resistance, in which the adhesive strength of the oxide layer is to be improved by adding zirconium, titanium and rare earth metals (lanthanides).

However, the oxidation resistance of the alloy described in this document is not sufficient for high application temperatures and high loads.

A further improvement in the oxidation resistance of iron-chromium-aluminum alloys with 4.5 - 6.5 mass% aluminum can be achieved if the content of rare earth metals is set at 0.06-0.15 mass%, as described in EP-A 0429793.

In GB-A 2070642 it is proposed to improve the oxidation resistance by adding rare earth metals, yttrium, hafnium, zirconium and titanium in the amount of altogether 2 mass%, and in EP-B 051 6 267 it is described how by adding Molybdenum in the amount of 4% by mass, the oxidation resistance can be increased without sacrificing ductility.

The alloys described in the cited documents no longer meet recent requirements for an iron-chromium-aluminum alloy with higher thermal stress while simultaneously reducing the foil or wire cross-section. A higher aluminum content is required to prevent early destruction of thin foils or wires by the so-called "breakaway corrosion". "Breakaway corrosion" is a special form of corrosion that occurs when, with very thin dimensions, due to the constant replication of aluminum oxide during the oxidation process, all of the aluminum in the material is consumed and no more aluminum is used to replicate the protective aluminum oxide layer is available.

The alloys listed in the cited documents also do not meet requirements for increased heat resistance and an increased electrical resistance compared to the prior art

Iron-chromium-aluminum alloys with an increased aluminum content of up to 10% by mass for use in thin foils are described in EP-A 0516097. The oxide layer adhesion in the alloy described is achieved by a combined addition of lanthanum and zircon. EP-A 0658632A 1 describes an alloy with up to 8% by mass of aluminum which maintains the adhesive strength of the oxide layer by adding the reactive elements V, Nb, Ta, Ti, Zr, Hf.

However, such alloys have considerable disadvantages, since alloys with additives of such reactive elements are sensitive to the so-called "overdoping", a selective internal corrosion which is caused by the reactive elements and is particularly sensitive to the alloys with aluminum contents above 6% by mass are. This selective internal corrosion leads to rapid destruction of the film.

DE-A 36 21 569 relates to the production of a chromium-aluminum-iron alloy for use as a support material for catalysts, the alloy 10 - 61 wt.% Chromium, 6 - 25 wt.% Aluminum and 0.001 - 1 each , 0 wt .-% lifespan additives. In addition to additions of Ca and / or Ba between 0.001 and 0.1% by weight, the alloy can also contain one or more of the elements or oxides Y, La, Ti, Zr, Hf, AbOa powder between 0.001 and 1.0 be added.

DE-AS 39 1 1 61 9 discloses a ductile, metallic semi-finished product in the form of foils or strips, which essentially consists of iron, 10-30% by weight chromium, 4-15% by weight aluminum and 0.005-1.0% by weight of life-increasing additives, such as rare earths, Y, Ti, Zr, Hf, Nb, Ca, Ba, Mg, the semi-finished product having a microscopic surface roughness of Ra> 0.3 μm at least on one side. The semi-finished product can be used as a support material in catalysts.

The invention has for its object an iron-chromium-aluminum alloy with a significantly reduced oxidation speed compared to the prior art, an increased service life (resistance to "breakaway corrosion), increased electrical resistance, high heat resistance and good deformability at high Resistance to to design selective internal corrosion (overdoping) and good adhesion of oxide layers.

This object is achieved by a ferritic iron-chromium-aluminum-yttrium-hafnium alloy with a long service life and high electrical resistance at high temperatures of the following composition (in mass%):

Cr: 16-22

AI: 6-10

Si: 0.02-1.0

Mn: max. 0.5

Hf: 0.02-0.1

Y: 0.02-0.1

Mg: max. 0.1

Ti: max. 0.02

Zr: max. 0.09

SE: max. 0.02

Sr: max. 0.1

Ca: max. 0.1

Cu: max. 0.5

V: max. 0.1

Ta: max. 0.1

Nb: max. 0.1

C: max. 0.03

N: max. 0.01

B: max. 0.01

Co: max. 2.0

W: max. 2.0

Mon: max. 2.0

Remainder iron as well as contamination due to melting

Advantageous developments of the subject matter of the invention can be found in the associated subclaims.

A preferred alloy has the following composition (in% by mass): Cr: 17-21

AI: 6 - 8.5

Si: 0.05-0.9

Mn: 0.1-0.4

Hf: 0.02-0.5

Y: 0.02 -0.5

Mg: 0.001-0.01

Ti: <0.001

Zr: <0.08

SE: <0.01

Sr: 0.1 max

Ca: max. 0.1

Cu: max.0.5

V: max.0.1

Ta: max. 0.1

Nb: max.0.1

C: 0.03 max

N: max. 0.01

B: max. 0.01

Co: <1.5

W: <1.7

Mo: <1.5

Remainder iron as well as contamination due to melting

According to a further idea of the invention, the contents of molybdenum, cobalt and tungsten are set as follows (in% by mass)

Co 0.1 - 1.4 W 0.1 - 1.6 Mo 0.1 - 1.5 where the sum of Co + W + Mo (in mass%) is 0.5 - 3.0.

The aluminum content is preferably set between 6.5-8.5 mass%. The alloy should contain zircon and carbon because it has been found that these elements help to ensure that the protective aluminum oxide layer is firmly anchored in the base material. This extends the life of the material. However, it is also important to avoid overdoping with zirconium, because otherwise pores will form in the vicinity of the anchorages, which will damage the foils. It is therefore advantageous to set the proportions of zircon in the range up to 0.09% by mass and of carbon in the range up to 0.03% by mass.

Also proposed is a process for the production of strips, wires and foils consisting of the alloy according to the invention by coating an iron-chromium steel, which contains reactive elements and further additives, with aluminum or an aluminum alloy and subsequent diffusion annealing.

In the same way, fabrics and knitted fabrics made of wires, tapes and foils can be produced from the alloy according to the invention.

Preferred fields of application of the alloy according to the invention are carrier films for metallic catalysts, in particular carrier films for electrically preheatable automotive exhaust gas catalysts, as heating conductors and as components in industrial furnace construction and in gas burners.

The advantageous properties of the alloy according to the invention are evident from the following exemplary embodiments.

Table 1 shows, by way of example, analyzes of batches from the alloy according to the invention (marked with "X") and the comparison alloys lying outside the composition according to the invention.

All alloy variants were made from cast blocks by hot rolling and then cold rolling at about 200 ° C. From the cold-rolled blocks were sampled for oxidation tests and hot tensile tests by rolling and / or machining; Wires for measuring electrical resistance were made by wire drawing.

Table 2 shows that the specific electrical resistance of the alloy according to the invention with values between 1.45 Ωmm ² / m and 1.6 Ωmm ² / m or above clearly exceeds the prior art. Table 2 also shows that there is cold formability as long as the aluminum content does not exceed 10% by mass.

The advantageous oxidation properties of the alloy according to the invention can be found in Table 3. For the tests, ground and cleaned test coupons of various test alloys were stored in air at 1 100 and 1 200 ° C for 1 056 hours. Every 96 hours during the test and after the end of the test, the mass change was determined gravimetrically and the depth of the internal oxidation was determined metallographically. The results summarized in Table 3 show that the alloy according to the invention has a reduced change in mass compared to the prior art (example R1) (as a measure of the rate of oxidation) and a small depth of internal corrosion. All examples corresponding to the alloy according to the invention are free of local internal corrosion (overdoping) and they show no flaking of oxide layers.

The long service life of foils of the alloy according to the invention compared to the prior art is shown in Figure 1. The diagram shows that with the same film thickness, the service life of the alloy according to the invention is approximately four times that of an alloy corresponding to the prior art (example R1). Alternatively, the film thickness of carrier films for metallic automotive catalytic converters can be reduced from 50 μm to less than 30 μm without reducing the service life. This long service life is achieved with an increase in the aluminum content and a precise coordination of the reactive elements yttrium, hafnium, titanium, zirconium and the rare earth metals (lanthanoids). These elements are required in certain concentrations to prevent the protective oxide layers from flaking off, but on the other hand they cause the so-called "overdoping" at high aluminum contents, a type of selective internal corrosion. Surprisingly, it has been shown that with a combination of the elements yttrium and hafnium and a limitation of the elements zirconium, titanium and the lanthanoids, the spontaneous formation of a protective layer of C.-AI2O3 begins. This layer is so thermodynamically stable that it can no longer be undergrown by internal oxidation.

Figure 2 shows the advantageous influence of yttrium and hafnium on the oxide layer. The yttrium / hafnium alloy according to the invention (E2) shows a thin oxide layer after the oxidation test without any signs of selective internal oxidation. The zirconium- and titanium-doped alloy (example D6) lying outside the invention, on the other hand, has a strong internal oxidation (overdoping). Figure 3 shows that the significantly lower oxidation depth of the alloy according to the invention exists over the entire temperature range between 900 and 1300 ° C.

Another advantageous property of the alloy according to the invention is its high hot tensile strength at elevated temperatures, as can be seen in FIG. 4. Surprisingly, it has been shown that the hot tensile strength can be increased by a factor of 2-3 by a targeted combination of the alloying elements molybdenum, tungsten and cobalt compared to the prior art.

The effect of the individual alloy components is described below: The chromium content of the alloy according to the invention is between 16 and 22% by mass in order to ensure adequate oxidation resistance and the desired electrical resistance. Higher chromium contents make it considerably more difficult to process iron-chromium-aluminum alloys.

The aluminum content of the alloy according to the invention should be between 6 and 10% by mass, preferably between 6.5 and 8.5% by mass, since the desired electrical resistance and the resistance to "breakaway corrosion" cannot be achieved with thin foils at lower aluminum contents. The aluminum content is limited to max. 10% limited, since forming is no longer possible with higher aluminum cases due to the formation of ordered intermetallic phases.

The silicon content of the alloy according to the invention is between 0.1 and 1% by mass, since the oxidation-inhibiting effect of silicon does not occur at lower silicon contents: at higher silicon contents, the occurrence of embrittling suicides and significant loss of ductility must be expected.

Manganese is limited to 0.5% by mass because this alloying element reduces the resistance to oxidation.

The hafnium content of the alloy according to the invention must be at least 0.02% by mass in order to ensure good adhesion of the oxide layers. However, it must not exceed 0.1% by mass, since internal corrosion can occur at higher contents. In addition to the hafnium, the alloy according to the invention must contain between 0.02 and 0.08% by mass, since the immediate formation of the protective α-Al ₂ 03 and the low rate of oxidation only occur when the two alloying elements act in combination. The yttrium content is reduced to 0.1 mass% limited to avoid the so-called "overdoping". In addition, the alloy contains 0.001-0.01 mass% magnesium.

An essential characteristic of the alloy according to the invention is the limitation of the contents of the reactive elements zirconium, titanium, strontium, calcium and the rare earth metals (lanthanoids). These elements have to be restricted because they suppress the formation of α-Abθ3 in iron-chromium-aluminum alloys with aluminum contents of 6% and more and can thus contribute to the selective internal oxidation. Since there is an additive effect of these elements, the total calcium + strontium + titanium + zirconium + rare earth metals must not exceed 0.05% by mass.

The elements vanadium, tantalum, niobium are each limited to a maximum of 0.1 mass%, copper to 0.5 mass%. In higher concentrations, these elements have undesirable effects on the oxide layer adhesion, the oxidation rate and the deformability of the iron-chromium-aluminum alloys.

Molybdenum can the alloy up to max. 1 mass% can be added. With higher molybdenum contents, a reduction in the service life can be expected. The alloy also contains cobalt up to 2% by mass and tungsten up to 2% by mass to ensure sufficient heat resistance. The sum of the alloy elements Mo + Co + W should be at least 0.5% by mass for sufficient hot tensile strength, but not more than 3% by mass in order to maintain the deformability.

The carbon content of the alloy according to the invention is limited to a maximum of 0.03 mass%, since the deformability is reduced at higher carbon contents. The nitrogen content is reduced to 0.01% by mass in order to avoid undesirable nitride formation. The levels of phosphorus and sulfur should be kept as low as possible since these surface-active elements reduce both the high-temperature corrosion resistance and the ductility of the alloy.

The alloy according to the invention can be used for tapes, foils and wires and for fabrics and knitted fabrics which are made from wires, tapes and foils. The advantageous properties of the alloy according to the invention come into play in particular in the case of thin film or wire cross sections.

The alloys according to the invention can be produced by continuous casting, thin slab casting, strip casting, wire casting or by block casting. Films and tapes made of the alloy according to the invention are preferably produced by roll-cladding or coating an Fe-Cr steel with an aluminum content between 0 and 5% and further additives on one or both sides with aluminum or an aluminum-silicon alloy and the composite material obtained in this way the final dimension or an intermediate dimension is rolled. Wires made from the alloy according to the invention can preferably be produced by coating a wire made of chromium steel with aluminum or an aluminum-silicon alloy and the composite material obtained in this way is made to the final or an intermediate dimension by wire drawing. A material with a homogeneous composition according to the invention is achieved both with drawn wires and with rolled foils by diffusion annealing at the final or intermediate dimensions.

Table 1: Composition of the alloy examples

Table 2: Electrical resistance (after cold working) and deformability of the alloy examples

Table 3: Results of the cyclical oxidation tests in dry air; Test duration: 1056 hours Cycle: 96 hours.

Claims

claims 1. Ferritic iron-chromium-aluminum-yttrium-hafnium alloy with a long service life and high electrical resistance at high temperatures of the following composition (in mass%): Cr: 16-22 AI: 6- 10 Si: 0.02-1.0 Mn: max.0.5 Hf: 0.02-0.1 Y: 0.02-0.1 Mg: 0.1 max Ti: 0.02 max Zr: 0.09 max SE: max.0.02 Sr: 0.1 max Ca: max. 0.1 Cu: max.0.5 V: max.0.1 Ta: max. 0.1 Nb: max.0.1 C: 0.03 max N: max. 0.01 B: max. 0.01 Co: max. 2.0 W: max. 2.0 Mon: max. 2.0 Remainder iron as well as contamination due to melting Alloy according to claim 1 characterized by the following composition (in mass%): Cr: 17-21 AI: 6 - 8.5 Si: 0.05-0.9 Mn: 0.1-0.4 Hf: 0.02-0.5 Y: 0.02-0.5 Mg: 0.001-0.01 Ti: <0.001 Zr: <0.08 SE: <0.01 Sr: max. 0.1 Ca: max. 0.1 Cu: max. 0.5 V: max. 0.1 Ta: max. 0.1 Nb: max. 0.1 C: max. 0.03 N: max. 0.01 B: max. 0.01 Co: <1, 5 W: <1, 7 Mo: <1, 5 Remainder iron as well as contamination due to melting 3. Alloy according to claim 1 or 2, characterized in that the contents of molybdenum, cobalt and tungsten are set as follows (in mass%): Co: 0.1-1.4 W: 0.1-1.6 Mo: 0.1-1.0, where the Sum of Co + W + Mo (in mass%) is 0.5 - 3.0. 4. Alloy according to claim 3, characterized in that the contents of molybdenum, cobalt and tungsten are set as follows (in% by mass) Co: max. 0.5 W: max. 1, 0 Mon: max. 0.5, where the Sum of Co + W + Mo (in mass%) is 0.5 to 2.0. 5. Alloy according to one of claims 1 to 4, characterized in that the aluminum content (in mass%) is between 6.5 and 8.5. 6. Alloy according to one of claims 1 to 5, characterized in that the sum of calcium + strontium + titanium + zirconium + rare earth metals (in mass%) is <0.05. 7. Alloy according to one of claims 1 to 6, characterized in that the sum of zirconium + yttrium + hafnium + rare earth metals (in mass%) is <0.2. 8. Alloy according to one of claims 1 to 7, characterized in that the sum of zirconium + carbon (in mass%) is <0.1. 9. Alloy according to one of claims 1 to 8, characterized by further additions (in mass%) of nickel in the range between 0 and 0.3. 10. A method for producing strips, wires and foils of the alloy according to claims 1 to 9 by coating an iron-chromium steel, which contains reactive elements and other additives, with aluminum or an aluminum alloy and subsequent diffusion annealing, the specific electrical resistance between 1, 45 Ωmm ² / m and 1, 6 Ωmm ² / m is set. 1 1. The method of claim 10, wherein fabrics and knitted fabrics are made of wires, tapes and foils. 12. Use of the alloy according to one of claims 1 to 9 as carrier foils for metallic catalysts, in particular as carrier foils for electrically preheatable automotive exhaust gas catalysts. 13. Use of the alloy according to one of claims 1 to 9 as an electrical heating conductor. 4. Use of the alloy according to one of claims 1 to 9 as components in industrial furnace construction and in gas burners.